This section describes the Packet Pair source generator; the relevant files are:
~ns/plm/cbr-traffic-PP.cc, ~ns/tcl/lib/ns-default.tcl. The OTcl class name of
the PP source is: Application/Traffic/CBR_PP.
The Packet Pair (PP) source generator is in the file
~ns/plm/cbr-traffic-PP.cc. This source
generator is a variation of the CBR source generator in ~ns/cbr_traffic.cc.
We just describe the salient differences between the code of
the CBR source and the code of the PP source.
The default values in ~ns/tcl/lib/ns-default.tcl for the PP source generator are the same
than for the CBR source. We need for the PP source generator a new parameter PBM_:

Application/Traffic/CBR_PP set PBM_ 2 #Default value;

The OTcl instvar bounded variable PBM_ (same name in C++ and in OTcl)
specifies the number of back-to-back packets to be sent. For PBM_=1 we
have a CBR source, for PBM_=2 we have a Packet Pair source (a source which
sends two packets back-to-back), etc. The mean rate of the PP source is
rate_, but the packets are sent in burst of PBM_ packets. Note
that we also use the terminology Packet Pair source and Packet Pair burst for
PBM_2.
We compute the next_interval as: